The promise of a broadband integrated services digital network has led to the study of mechanisms for efficient transport of real-time compressed data (especially video data) in packet switching networks. Two principal approaches have emerged. One approach is for the network to offer guarantees of constant bandwidth to each data connection--Such connections are sometimes referred to as constant bit rate transmission channels. An alternative approach is for the network to provide information via variable bit rate transmission channels. This approach utilizes network resources more efficiently than constant bit rate channels by exploiting packet-switching and the inherent variations or bursts of the data being transmitted (such variations are particularly evident in video data). Variable bit rate channels can provide a constant quality signal to end users, assuming that the values of the encoding parameters remain constant and there is no loss of information due to packet losses. However, the bandwidth provided by a packet-switched network is in general time-varying. As long as a network is lightly loaded, it has the bandwidth available to carry the variable bit rate stream at a constant quality, however, under heavy loads there could be loss of information due to buffer overflows or excessive data delays. The degradation of signal quality due to such overloads can be reduced by providing congestion control mechanisms.
ATM networks, a specific class of variable bit rate transmission channels, impose special constraints on the choice of control mechanisms, mainly due to the small fixed cell size. The small cell size places a premium on the number of bits that may be used in the header for control purposes. With respect to congestion control, the small ATM header size has meant that there is only one bit, called the Forward Explicit Congestion Notification ("FECN") bit, available for indicating congestion information in every data cell going in the forward direction (i.e. from a sender to a receiver). The small ATM cell size also reduces the cell processing time available at a switch, and thereby effectively limits the use of sophisticated algorithms at a switch for the purpose of congestion avoidance.
As a result, there is still much debate on the choice of appropriate congestion control mechanisms for data and video traffic in ATM networks. Two approaches for providing network congestion control were disclosed by K. K. Ramakrishan and R. Jain in "A binary feedback scheme for congestion avoidance in computer networks," ACM Transactions on Computer Systems, 8(2), pages 158-181, May 1990. In one of the approaches a switch is adapted to set a single bit in a cell header to indicate congestion when the average queue length at the switch at the time the cell arrives is greater than or equal to a particular fixed threshold. The other approach disclosed by Ramakrishan and Jain employs a hysteresis algorithm that involves two thresholds. A single bit within a cell header is set to indicate congestion after the average queue occupancy crosses the higher threshold, and once this higher threshold is crossed, the cell header bit continues to be set until the average queue occupancy reaches a lower threshold.
Another single bit based feedback control scheme is described in co-pending and commonly assigned U.S. patent application Ser. No. 118,531, "Method For Adaptive Control Of Windows And Rates In Networks," which was filed on Sep. 8, 1993, now U.S. Pat. No. 5,426,635 issued Jun. 20, 1995. This application shows a system wherein each switch along a path in a virtual circuit periodically provides queue occupancy information by setting a single bit in response to the instantaneous queue occupancy level. This set bit provides information used to adjust the amount or rate of information being sent to the switch by the sources within the circuit.
Unfortunately, these prior one-bit feedback schemes fail to provide information reflective of anything more than the instantaneous level of a monitored parameter being above or below a particular threshold.